This invention relates to an artificial foot and an ankle part connected by means of a two-part forked joint. The lower part of the latter which contains the articulation axis is screwed immovably into the foot part, separated by a plastic socket that is also firmly bonded to the foot part. The upper portion of the ankle joint which appears nearly U-shaped in a cross-sectional view and is made of a viscous, elastic material, grips the joint axis from below. It is immovably screwed into the ankle part, separated by a plastic socket that is also firmly bonded to the ankle.
In the socket of the foot part a ring-shaped articulator (which should be made of elastic material, preferably rubber) is loosely inserted to correspond to the dome-shaped articulation surface of the lower end of the ankle. The lower part of the ankle projects through the ring hole of the rubber articulator.
The present invention is an improvement of the previously known artificial foot joint disclosed in German Patent Specification No. 1,173,612. In the previous model the ring-shaped articulator, besides providing a rotation surface for the articulating surface of the ankle, also includes a dorsal projection in front and a rear projection for the plantar flexion. The disadvantage of this model lies in the absence of adaptability of the plantar flexion to the wearer of the prosthesis in regard to weight, age, etc.
In the previous model the sockets are discrete plastic molds that have been glued onto the corresponding surfaces of the wooden foot and ankle parts. This solution requires relatively complicated lathe work of the wooden parts as well as excessive space needs. The fastening of the plastic molds is difficult as they must be made of smooth, low friction material which is not suitable for gluing. Moreover, these molded plastic parts require excessive effort to place them in predetermined positions with sufficient precision.
Underneath the plastic socket of the foot part, the wood must have a sufficiently wide diameter to carry the transferred weight without failure. Because of the addition of the plastic part on top of this section of the wooden part, the articulation axis is relatively high. For an optimal approximation of natural conditions, however, it is in principle desirable to achieve the lowest possible position of the axis.
A further disadvantage is presented by the complicated production method required for the ball-shaped articulation surface of the wooden ankle. The lathe work for this surface must be extremely precise so that the plastic socket can be fitted tightly around it.
From a cosmetic point of view there is an additional disadvantage in that there is a visible gap in the joint between the foot and the ankle.